Field of the Invention
This invention relates generally to a system for the drying of a wet web of fibrous material on a rotary dryer. More specifically it relates to the control of the input of the hot air used in a drying system to protect (1) the rotary dryer from thermal stress caused by sudden temperature changes and (2) the air pervious support carrying the web through the dryer from high temperatures which can damage or destroy it.
In a throughdrying system operating with air heated to temperatures of 400.degree. F. and higher, the loss of the wet web during the drying step can (1) cause severe thermal stress to the metal parts of the dryer, specifically to the air pervious rotating cylindrical surface of the dryer, and (2) melt the traveling air pervious support, e.g., a polyester carrier fabric.
The result of either of these events can be catastrophic. In the case of thermal stress to the metal surfaces of the dryer, the result can be the distortion or even the rupture of the cylindrical surface resulting in a major shutdown until the dryer can be replaced or the surface restored.
In the case of a melted support, replacement will be required due to the sealing off of portions of the support rendering it unsuitable for use; and in the more severe case where the support is severely melted, the result can be severe sticking to the exterior cylindrical, air pervious surface of the rotating drum, clogging the surface. Again, a major shutdown--in this situation to clean out the dryer surface--is required.
Conventional temperature control systems tend to be slow acting. For this reason they are unsatisfactory when confronted with the need for an immediate response to prevent damage such as described above. Systems are also presently available which rely on water sprays, turning the burner down or turning the burner off to protect the air pervious support. However, water sprays can cause high levels of thermal stress for the dryer and can also wrinkle the air pervious support; turning the burner off requires that the complete system be cooled to allow purging prior to relighting. A system then which will avoid these problems and provide a rapid response time is highly desirable. Ideally, the system should also allow testing, start-up and trouble shooting of the dryer without subjecting either (1) the exterior cylindrical, air pervious surface of the dryer or (2) the air pervious support, to air heated to excessively high temperatures.
The subject invention is directed then to a system for overcoming or minimizing the difficulties described above when upsets occur in the drying system which could lead to thermal stress of the dryer itself and/or heat damage to the air pervious support carrying the web of fibrous material through the dryer. Additionally, this invention is directed to a system wherein during start-up, trouble shooting and testing, the heater means can be fired independently of the air system.